Back to EveryPatent.com
United States Patent |
6,211,325
|
Sun
,   et al.
|
April 3, 2001
|
High strength plastic from reactive blending of starch and polylactic acids
Abstract
Biodegradable polymers for use in forming high strength, degradable
plastics and methods of forming the polymers are provided. Broadly, the
methods comprise forming and heating a blended mixture of polylactic acid,
a starch, and a linkage group for joining or copolymerizing the polylactic
acid and starch. Preferred linkage groups comprise an isocyanate moiety,
with diphenylmethylene diisocyanate, hexamethylene diisocyanate, and
isophorone diisocyanate. The reacted mixture can then be formed into the
desired final product which has high tensile strength, modulus of
elasticity, percent elongation, and thermal stability.
Inventors:
|
Sun; Xiuzhi S. (Manhattan, KS);
Seib; Paul (Manhattan, KS);
Wang; Hua (Manhattan, KS)
|
Assignee:
|
Kansas State University Research Foundation (Manhattan, KS)
|
Appl. No.:
|
549439 |
Filed:
|
April 14, 2000 |
Current U.S. Class: |
528/66; 528/73; 528/361 |
Intern'l Class: |
C08G 018/10; C08G 018/28; C08G 063/06 |
Field of Search: |
523/128
528/66,73,361
|
References Cited
U.S. Patent Documents
5714230 | Feb., 1998 | Kameoka et al. | 428/141.
|
5753724 | May., 1998 | Edgington et al. | 523/124.
|
5763098 | Jun., 1998 | Kameoka et al | 428/481.
|
5961906 | Oct., 1999 | Muller et al. | 264/109.
|
Foreign Patent Documents |
0530987 | Aug., 1991 | EP.
| |
Other References
S. Jacobson et al., "Filling of Poly(Lactic Acid) With Native Starch",
Polymer Engineering and Science, vol. 36, No. 22, pp. 2799-2804, Nov.
1996.
|
Primary Examiner: Sanders; Kriellion
Attorney, Agent or Firm: Hovey, Williams, Timmons & Collins
Claims
We claim:
1. A method of preparing a biodegradable polymer composition utilized in
forming plastic articles, said method comprising the steps of:
forming a mixture comprising starch, polylactic acid, and a linkage group
comprising an isocyanate moiety; and
causing said starch, polylactic acid, and linkage group to react to yield
the polymer.
2. The method of claim 1, wherein said causing step comprises heating said
mixture to at least about 150.degree. C.
3. The method of claim 1, wherein said polylactic acid has an average
molecular weight of at least about 70,000 Daltons.
4. The method of claim 1, wherein said starch is selected from the group
consisting of corn starch, wheat starch, sorghum starch, potato starch,
and tapioca starch.
5. The method of claim 1, wherein said linkage group is selected from the
group consisting of compounds comprising two isocyanate moieties.
6. The method of claim 1, wherein said heating step is carried out for at
least about 2 minutes.
7. The method of claim 1, wherein said mixture comprises less than about 4%
by weight of said linkage group, based upon the total weight of the
mixture taken as 100% by weight.
8. The method of claim 1, wherein the weight ratio of starch:polylactic
acid is from about 1:99 to about 70:30.
9. The method of claim 1, wherein said forming step comprises forming a
precursor mixture comprising respective quantities of polylactic acid and
said linkage group and mixing said precursor mixture with said starch and
a second quantity of said polylactic acid.
10. The method of claim 9, wherein said polylactic acid is present in said
precursor mixture at a level of from about 96-99.9% by weight, based upon
the total weight of the precursor mixture taken as 100% by weight.
11. The method of claim 1, wherein said forming step comprises forming a
precursor mixture comprising respective quantities of polylactic acid,
starch, and said linkage group, and mixing said precursor mixture with
respective second quantities of starch and polylactic acid.
12. The method of claim 11, wherein said precursor mixture comprises from
about 30-99% by weight polylactic acid, from about 1-70% by weight starch,
and from about 0.1-4% by weight of said linkage group, said percentages by
weight being based upon the total weight of the precursor mixture taken as
100% by weight.
13. The method of claim 1, wherein the tensile strength of the polymer is
at least about 50 MPa.
14. The method of claim 1, wherein the percent elongation of the polymer is
at least about 3%.
15. The method of claim 1, wherein the modulus of elasticity of the polymer
is at least about 1500 MPa.
16. The method of claim 1, wherein the crystallinity of the polymer is at
least about 2 times greater than the crystallinity of polylactic acid.
17. The method of claim 1, wherein the melting point of the polymer varies
by less than about 3.degree. C. after four heating cycles when compared to
the first heating cycle.
18. A biodegradable polymer composition utilized in forming plastics, said
composition comprising a starch and polylactic acid reaction product, said
composition having a tensile strength of at least about 50 MPa.
19. The polymer of claim 18, wherein said starch and polylactic acid
reaction product comprises a starch molecule joined with a polylactic acid
molecule via a linkage group.
20. The polymer of claim 19, wherein said linkage group comprises two
isocyanate moieties.
21. The polymer of claim 20, wherein said linkage group is selected from
the group consisting of diphenylmethylene diisocyanate and hexamethylene
diisocyanate.
22. The polymer of claim 18, wherein said starch is selected from the group
consisting of corn starch, wheat starch, sorghum starch, potato starch,
and tapioca starch.
23. The polymer of claim 18, wherein the weight ratio of starch:polylactic
acid in said starch and polylactic acid reaction product is from about
1:99 to about 70:30.
24. The polymer of claim 19, wherein said polymer comprises less than about
4% by weight of said linkage group, based upon the total weight of the
polymer taken as 100% by weight.
25. The polymer of claim 18, wherein the modulus of elasticity of said
polymer is at least about 1500 MPa.
26. The polymer of claim 18, wherein the percent elongation of the polymer
is at least about 3%.
27. The polymer of claim 18, wherein the crystallinity of the polymer is at
least about 2 times greater than the crystallinity of polylactic acid.
28. A biodegradable polymer composition utilized in forming plastics, said
composition comprising a starch and polylactic acid reaction product, said
polymer having a modulus of elasticity of at least about 1500 MPa.
29. The polymer of claim 28, wherein said starch and polylactic acid
reaction product comprises a starch molecule joined with a polylactic acid
molecule via a linkage group.
30. The polymer of claim 29, wherein said linkage group comprises two
isocyanate moieties.
31. The polymer of claim 30, wherein said linkage group is selected from
the group consisting of diphenylmethylene diisocyanate and hexamethylene
diisocyanate.
32. The polymer of claim 28, wherein said starch is selected from the group
consisting of corn starch, wheat starch, potato starch, and tapioca
starch.
33. The polymer of claim 28, wherein the weight ratio of starch:polylactic
acid in said starch and polylactic acid reaction product is from about
1:99 to about 70:30.
34. The polymer of claim 29, wherein said polymer comprises less than about
4% by weight of said linkage group, based upon the total weight of the
polymer taken as 100% by weight.
35. The polymer of claim 28, wherein the modulus of elasticity of said
polymer is at least about 1500 MPa.
36. The polymer of claim 28, wherein the percent elongation of the polymer
is at least about 3%.
37. The polymer of claim 28, wherein the crystallinity of the polymer is at
least about 2 times greater than the crystallinity of polylactic acid.
38. A biodegradable polymer composition utilized in forming plastics, said
composition comprising:
a quantity of polylactic acid including a plurality of recurring monomers;
a quantity of starch including a plurality of recurring monomers; and
a linkage group bonded to at least one of said polylactic acid monomers and
to at least one of said starch monomers.
39. The polymer of claim 38, wherein said polylactic acid has an average
molecular weight of at least about 70,000 Daltons.
40. The polymer of claim 38, wherein said starch is selected from the group
consisting of corn starch, wheat starch, sorghum starch, potato starch,
and tapioca starch.
41. The polymer of claim 38, wherein said linkage group comprises two
isocyanate moieties.
42. The polymer of claim 41, wherein said linkage group is selected from
the group consisting of diphenylmethylene diisocyanate and hexamethylene
diisocyanate.
43. The polymer of claim 38, wherein said polymer comprises less than about
4% by weight of said linkage group, based upon the total weight of the
polymer taken as 100% by weight.
44. The polymer of claim 38, wherein the weight ratio of starch:polylactic
acid in said polymer is from about 1:99 to about 70:30.
45. The polymer of claim 38, wherein the tensile strength of the polymer is
at least about 50 MPa.
46. The polymer of claim 38, wherein the percent elongation of the polymer
is at least about 3%.
47. The polymer of claim 38, wherein the modulus of elasticity of the
polymer is at least about 1500 MPa.
48. The polymer of claim 38, wherein the crystallinity of the polymer is at
least about 2 times greater than the crystallinity of polylactic acid.
49. A plastic product formed from a polymer according to claim 38.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is broadly concerned with modified synthetic polymer
compositions for use in forming high strength, biodegradable plastics and
methods of forming such compositions. More particularly, the inventive
compositions comprise polylactic acid joined or copolymerized with starch
via a linkage or compatabilizing group which comprises a diisocyanate
moiety. The invention allows lower quantities of polylactic acid to be
utilized while maintaining the biodegradability of polylactic acid-derived
plastics and improving their mechanical properties.
2. Description of the Prior Art
Plastics (synthetic resins) are widely used and an important material in
current commercial products. As more plastics having varying mechanical
properties are developed, industries are finding uses for plastics which
only a few years ago would have been unexpected. For example, many
automobiles which were previously formed entirely of metal now include
plastic components such as plastic body panels. Furthermore, these plastic
components are so well-designed that it is generally difficult to visually
differentiate between the plastic and steel components on an automobile.
In addition to automobile components, plastics find uses in innumerable
products including children's toys, kitchen dishes and appliances,
packaging materials, and medical products.
While plastics have generally been an inexpensive and efficient material
for manufacturing products, they are derived in large part from petroleum
resources which are finite and increasing in cost. Thus, it is important
to develop new methods and materials for forming plastics as an
alternative to the current methods.
Additionally, the environmental impact of discarded plastic objects is of
growing global concern due to the fact that disposal methods for such
wastes are quite limited. Incineration of the plastic wastes generates
toxic air pollution. At the same time, satisfactory landfill sites are
limited, and most durable plastics do not biodegrade. There is, thus, a
need for durable and biodegradable plastic materials, particularly for
short-term use items such as packaging materials and disposable utensils.
Fully biodegradable, synthetic polymers have been commercially available
for several years. Such polymers include polylactic acid (PLA),
polycaprolactone (PCL), and polyhydroxybutyratevalerate (PHBV). Among
these polymers, PLA has been extensively studied in medical implants,
sutures, and drug delivery systems. Unlike other available biodegradable
synthetic polymers, PLA exhibits promising mechanical properties, thus
making it appealing for use as a disposable and biodegradable plastic
substitute. However, PLA is costly compared to conventional
petroleum-based plastics, and its degradation rate is slow compared to the
waste accumulation rate. Finally, another disadvantage of PLA is that its
modulus of elasticity decreases by about 85% at temperatures above its
glass transition temperatures (60.degree. C.), where it becomes very soft,
and thus it has only limited applications.
Starch is a renewable and degradable carbohydrate biopolymer that can be
purified from various sources by environmentally sound processes. By
itself however, starch has severe limitations in plastic applications due
to its water solubility. That is, articles molded from starch will swell
and deform upon exposure to moisture. To decrease interaction with water,
starch is often blended with hydrophobic polymers which reduce the use of
petroleum polymers while simultaneously increasing the biodegradability of
the product.
Starch has been blended with synthetic biodegradable polymers such as PCL,
PHBV, and poly(hydroxybutyrate) (PHB). However, starch and PLA have not
previously been successfully blended because they are immiscible polymers.
There is a need for a compatibilizer which can enhance the compatibility
of starch and PLA to yield a high-strength biodegradable plastic.
SUMMARY OF THE INVENTION
The present invention overcomes the problems of the prior art by providing
novel polymer compositions useful for forming high-strength, degradable
plastics. The inventive compositions broadly comprise starch reacted with
polylactic acid via compatibilizing or linkage groups.
In more detail, polymer compositions according to the invention are
prepared by forming a mixture comprising the starch, polylactic acid, and
linkage group and causing the ingredients of the mixture to react such as
by heating the mixture to a temperature of at least about 150.degree. C.,
and preferably at least about 175.degree. C. The heating step should be
carried out for at least about 2 minutes, and more preferably from about
3-5 minutes.
The weight ratio of starch:polylactic acid in the mixture should be from
about 1:99 to about 70:30, preferably from about 40:60 to about 60:40, and
more preferably from about 45:55 to about 50:50. The average molecular
weight of the polylactic acid used to prepare the mixture is preferably at
least about 70,000 Daltons, and more preferably from about 90,000-140,000
Daltons. Suitable starches include those selected from the group
consisting of corn starch, wheat starch, sorghum starch, potato starch,
tapioca starch, or any other starch from crops and plants.
The linkage group should comprise at least one isocyanate moiety, and more
preferably at least two such isocyanate moieties, with preferred linkage
groups being selected from the group consisting of diphenylmethylene
diisocyanate, hexamethylene diisocyanate, and isophorone diisocyanate. The
most preferred linkage group is diphenylmethylene diisocyanate. The
linkage group should be mixed with the starch and polylactic acid at a
level of less than about 4% by weight, preferably from about 0.1-2% by
weight, and more preferably from about 0.2-0.5% by weight linkage group,
based upon the total weight of the starch/polylactic acid/linkage group
mixture taken as 100% by weight.
In forming the starch/polylactic acid/linkage group mixture, it is
preferred that all of the ingredients simply be mixed together.
Alternately, a precursor mixture comprising respective quantities of
polylactic acid and of the linkage group is formed, and the precursor
mixture is then mixed with the starch and the remainder of the polylactic
acid. In these instances, the polylactic acid should be present in the
precursor mixture at a level of from about 96-99.9% by weight, and
preferably from about 98-99% by weight, based upon the total weight of the
precursor mixture taken as 100% by weight.
In another embodiment, a precursor mixture comprising respective quantities
of polylactic acid, starch, and the linkage group is formed, and the
precursor mixture is then mixed with the remainder of the starch and
polylactic acid. In this embodiment, the precursor mixture should comprise
from about 30-99% by weight polylactic acid, from about 1-70% by weight
starch, and from about 0.1-4% by weight of the linkage group. Even more
preferably, the precursor mixture should comprise from about 30-70% by
weight polylactic acid, from about 30-70% by weight starch, and from about
1-2% by weight of the linkage group, based upon the total weight of the
precursor mixture taken as 100% by weight.
The final prepared polymer composition can then be used to form a plastic
in the same manner as prior art plastic-forming processes, including
utilizing known additives and plasticizers. For example, the polymer
composition can be formed into disposable food utensils, packaging for
food, and numerous other plastic items. Advantageously, the inventive
methods allow smaller quantities of polylactic acid to be utilized, thus
decreasing the cost of the final product compared to prior art plastic
products derived from polylactic acid. Furthermore, by using starch with
smaller quantities of polylactic acid rather than simply large quantities
of polylactic acid alone, the biodegradability of the polylactic acid is
not compromised.
The inventive compositions, and the plastics derived therefrom, have highly
desirable mechanical properties in general, and have substantially
improved mechanical properties when compared to pure polylactic acid or to
prior art polylactic acid-derived plastics. For example, the ASTM D638-92
tensile strength of the inventive polymer composition is at least about 50
MPa, preferably at least about 60 MPa, and more preferably from about
40-75 MPa. The ASTM D638-92 percent elongation of the composition is at
least about 3%, preferably at least about 4%, and more preferably from
about 3-6%. Additionally, the ASTM D638-92 modulus of elasticity of the
composition is at least about 1500 MPa, preferably at least about 1800
MPa, and more preferably at least about 1800-2000 MPa.
The crystallinity (X.sub.c, described in detail below) of the compositions
is at least about 2 times, and more preferably at least about 5 times
greater than the crystallinity of pure polylactic acid (i.e., polylactic
acid which has not been blended with some other polymer or modifier).
Finally, when a polymer composition according to the invention is
subjected to four heating cycles (i.e., it is heated to its melting
temperature followed by cooling to room temperature, and this cycle is
then repeated three times for a total of four cycles) the melting point of
the composition during the fourth heating cycle is within about 3.degree.
C. of the composition melting point during the first heating cycle. Thus,
the melting point of the composition during the fourth heating cycle is
preferably 170-175.degree. C.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an FTIR spectrum of activated polylactic acid (PLA) produced by
hot blending 1% MDI with the PLA;
FIG. 2 is an FTIR spectrum of pure PLA; and
FIG. 3 is a graph comparing the dynamic storage modulus of pure PLA to a
sample comprising starch and PLA blended with MDI.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
EXAMPLES
The following examples set forth preferred methods in accordance with the
invention. It is to be understood, however, that these examples are
provided by way of illustration and nothing therein should be taken as a
limitation upon the overall scope of the invention.
Example 1
Materials
Polylactic acid (PLA) having a molecular weight of about 120,000 Da and
polymerized primarily from L-lactic acid was obtained from Shimadzu, Inc.
(Japan). The glass transition temperature of the PLA was about 60.degree.
C., and the peak melting temperature was about 172.degree. C.
Wheat starch (Midsol 50) was obtained from Midwest Grain Products, Inc.
(Atchison, Kans.). The wheat starch granules contained approximately 75%
amylopectin and 25% amylose. The moisture content of the starch was about
11.9%. The starch was oven-dried to a moisture content of about 1% prior
to blending with the PLA.
Experimental Design
In this example, three experiments were conducted. In one experiment
(hereinafter referred to as "Experiment I"), the PLA was blended with a
small amount of diphenylmethylene diisocyanate (MDI) to produce PLA
reacted with MDI. Because MDI was used in molar excess to PLA, it was
assumed to contain some reactive isocyanate groups and is hereinafter
referred to as "activated" PLA. A small amount of the activated PLA was
then added to a mixture of starch and PLA and blended. It is believed that
the activated PLA interacted with the starch due to the attached
isocyanate group, resulting in the PLA's improved compatibility with
starch.
In another experiment (hereinafter referred to as "Experiment II"), starch
and PLA were blended with a small amount of MDI to produce a "diblock"
after which a small amount of the diblock was added to the bulk mixture of
starch and PLA. It is believed that the diblock enhanced the compatibility
of the starch and PLA.
In a final experiment (hereinafter referred to as "Experiment III"), a
small amount of MDI was added to the bulk mixture of starch and PLA, then
blended together (i.e., a direct blending method). It is believed that the
diblock was formed in situ during blending, resulting in high starch-PLA
compatibility.
In each of the above Experiments I-III, the components were pre-mixed under
ambient conditions using a general mixer (Kitchen Aid.RTM.). All reactive
blending was performed by using an intensive mixer (Rheomix 600, Haake,
Paramus, N.J.) at 190.degree. C. and 135 rpm for four minutes. The weight
ratio of starch to PLA in each of Experiments I-III was 45:55.
Tensile Bar Preparation
Each of the prepared blends were individually compression-molded into
tensile bars following ASTM D686-92 and using a Carver Hot Press (3890,
Auto "M," Carver, Inc., Wabash, Ind.) at a molding temperature of
176.degree. C. and a molding pressure of 4.2 MPa for 9-15 minutes. The
molded specimens were cooled to room temperature and preconditioned at 55%
relative humidity (RH) and 22.degree. C. for at least 48 hours before
mechanical analyses.
Testing Methods
The mechanical properties of the tensile bars were measured using an
Instron testing system (Model 4466, Canton, Mass.) according to the
Standard Test Method for Tensile Properties of Plastics (ASTM D638-92).
The stress and the percent of strain at the maximum stress were
determined.
Differential scanning calorimetry (DSC) analyses were performed to
determine the thermal transition measurements (DSC7, Perkin-Elmer,
Norwalk, Conn.). The DSC instrument was calibrated using the melting
temperature and enthalpy of standard material indium. The dried, ground,
blended samples were weighed into an aluminum pan and sealed hermetically.
An empty pan was used as a reference. The DSC temperature was increased
from 25 to 200.degree. C. at a rate of 10.degree. C./min.
The sum of the enthalpy of the blends at different thermal stages was used
as an estimate of the crystallinity (X.sub.c) using the following equation
from Fisher et al., Kolloid--Z.U.Z. Polym., 251:980-990 (1972):
X.sub.c (%)=(.DELTA.H.sub.m +.DELTA.H.sub.ci).multidot.100/(93
J/g.multidot.X.sub.PLA),
where: .DELTA.H.sub.m and .DELTA.H.sub.ci are the enthalpies (J/g) of
endotherm of melting and exotherm of crystallization of the blend,
respectively; 93 J/g is the enthalpy of fusion of a PLA crystal of
infinite size; and X.sub.PLA is the PLA content (in percent by weight,
based upon the total weight of the sample taken as 100% by weight).
The dynamic mechanical properties were determined using a dynamic
mechanical analyzer (DMA-7e, Perkin-Elmer) with a 3-point bending
rectangle method at 1 Hz. Storage (G') was analyzed, and the DMA
temperature range was increased from 25 to 180.degree. C. at a rate of
5.degree. C./min.
A spectrometer (KBr plate, ATI Mattson Research Series 1 FTIR, Madison,
Wis.) was used to obtain infrared spectra of selected samples.
Results and Discussion
1. Experiment I
The data for Experiment I is set forth in Table 1. The tensile strength of
the blends with more than 1% MDI and the activated PLA was significantly
higher than the blends without the activated PLA. Furthermore, the
elongation of the blends improved as the MDI content increased
significantly up to 1%. The blend with 10% of the activated PLA containing
1.05% MDI had mechanical properties similar to those of the blend with 5%
of the activated PLA containing 2.1% MDI, and the tensile strength was
improved by about 54% when compared to the control. This indicates that
reactions or interactions between the isocyanate groups from MDI and the
hydroxyl or carbonyl groups from PLA or starch likely occur during
blending.
TABLE 1
Mechanical properties of starch and PLA blends at 25.degree. C., blends
prepared in the presence of activated PLA developed from MDI and one
homopolymer PLA.sup.a.
Blends
MDI in PLA Activated PLA Starch:PLA.sup.c Tensile Elongation
% (w/w) %.sup.b (w/w) strength MPa %
0 0 0:100 62.3 .+-. 2.5 5.1
0 0 45:55 33.0 .+-. 1.6 2.4
0.42 5 45:55 34.3 .+-. 3.1 2.5
1.05 5 45:55 44.7 .+-. 1.4 3.3
1.05 10 45:55 49.8 .+-. 2.7 4.1
2.1 5 45:55 51.2 .+-. 4.9 4.2
.sup.a Values reported here are averages of five tensile bar tests.
.sup.b Percent by weight based upon total weight of blend taken as 100% by
weight.
.sup.c Calculated by including the PLA from the activated PLA.
FIGS. 1 and 2 depict FTIR spectra obtained during this experiment. Three
small peaks assigned to amide groups were observed in the spectra.
Referring to FIG. 1 which depicts the FTIR spectrum of the activated PLA
formed with 1.05% MDI, two small peaks were observed at 1650 and 1526 wave
numbers and another peak was observed at 3420 wave numbers, indicating two
different stretching vibrations. The peaks at 1650 and 1526 were not
present in the pure PLA sample, and the peak at 3420 was not as strong in
the pure PLA sample (see FIG. 2). The two peaks at 1650 and 1526 were
close to 1655 and 1550 for secondary amide groups associated with large
groups attached to a carbonyl, and the peak at 3420 was between 3440
(free) and 3300 (associated) amide groups for the .nu.NH stretching
vibration as reported by Nakanishi et al., Infrared Absorption
Spectroscopy, 2nd ed., Holden-Day, Inc., San Francisco, Calif. (1977).
Therefore, it is believed that these three peaks are due to amide or
urethane groups formed in situ during blending with PLA.
Again, referring to FIG. 1, a large peak at about 2300 wave numbers was the
isocyanate group from MDI which remained after partial reaction with the
PLA. The remaining isocyanate groups were expected to react with the
hydroxy groups of starch and PLA during blending. The activated PLA had
higher thermal stability compared to the pure PLA. The melting
temperatures of the pure PLA decreased by about 10.degree. C. after four
heating cycles, while they remained almost constant for the activated PLA
(see Table 2).
TABLE 2
Melting temperatures of pure PLA and activated PLA with 1% MDI
determined by heating cycles in the DSC.
T.sub.m ending
Samples T.sub.m starting (.degree. C.) T.sub.m at the peak (.degree.
C.) (.degree. C.)
Pure PLA:
1st heating 154.8 171.1 181.0
2nd heating 159.9 170.9 178.3
3rd heating 155.4 167.2 174.3
4th heating 141.5 163.1 170.9
Reactive PLA:
1st heating 159.2 175.7 183.9
2nd heating 159.6 175.3 183.1
3rd heating 160.1 175.1 183.6
4th heating 156.9 175.3 183.1
2. Experiment II
Table 3 sets forth the data from this example. Both the tensile strength
and the elongation of the blends increased as the MDI content in the
blends increased and were significantly higher than those of the control
blend. At fixed MDI content in the blend, the samples containing activated
PLA gave higher tensile strengths and elongations than those containing
the pre-formed diblock (see Tables 1 and 3).
TABLE 3
Mechanical properties of starch and PLA blends in the presence of
compatibilizer (diblock) developed by blending MDI with the two
homopolymers (starch and PLA).sup.a.
Blends
MDI in S/P.sup.b Diblock %.sup.c Tensile strength Elongation
% (w/w) % (w/w) Starch:PLA.sup.d MPa %
0 0 0:100 62.3 .+-. 2.5 5.1
0 0 45:55 33.0 .+-. 1.6 2.4
1.05 5 45:55 39.3 .+-. 2.2 2.7
2.06 5 45:55 42.8 .+-. 3.2 3.5
2.06 9.8 45:55 48.0 .+-. 3.2 3.8
3.26 5 45:55 45.0 .+-. 3.9 3.4
.sup.a Values reported here are averages of five tensile bar tests.
.sup.b S/P = Starch and PLA (45/55,w/w) diblock.
.sup.c % by weight based upon the total weight of the blend taken as 100%
by weight.
.sup.d Calculated by including the starch and PLA from the diblock.
3. Experiment III
In this experiment, the MDI was added directly to the starch and PLA
blending system, significantly improving the mechanical properties of the
blends (see Table 4). The blend with 0.5% MDI had the highest tensile
strength and modulus of elasticity, both of which were even higher than
that of pure PLA. The blend with 1% MDI gave the highest elongation among
the blends, which was slightly lower than that of the pure PLA. No
significant differences in mechanical properties were observed between the
1% and 2% MDI treatments.
TABLE 4
Mechanical properties of starch and PLA blended in the presence of MDI
(direct blending).sup.a.
Tensile
Starch:PLA strength Elongation
MDI %.sup.b (w/w) (MPa) % Modulus (MPa)
0 0:100 62.3 .+-. 2.5 5.1 .+-. 0.35 1536 .+-. 83
0 45:55 35.7 .+-. 1.6 2.6 .+-. 0.27 1720 .+-. 125
0.25 45:55 61.6 .+-. 3.5 4.3 .+-. 0.55 1870 .+-. 88
0.5 45:55 66.6 .+-. 3.3 4.4 .+-. 0.3 1972 .+-. 161
1 45:55 64.9 .+-. 2.6 4.8 .+-. 0.59 1941 .+-. 90
2 45:55 65.3 .+-. 3.4 4.5 .+-. 0.31 1920 .+-. 72
.sup.a Values reported here are averages of five tensile bar tests.
.sup.b Percent by weight based upon the total weight of the MDI/starch/PLA
mixture taken as 100% by weight
The molecular weight of MDI is quite small compared to the respective
molecular weights of the PLA and starch. At 1% MDI by weight in a blend of
starch:PLA at a weight ratio of 45:55, the mole ratio of MDI to PLA was
about 10, which would provide sufficient isocyanate groups to react with
all hydroxy or carboxyl groups. These results indicate that diblocks
linking starch to PLA were randomly formed in situ during blending, and
the diblocks then enhanced starch and PLA compatibility. The activated PLA
and diblocks produced in Experiments I and II as discussed above might
have been degraded due to exposure to excess thermal processing, resulting
in a lower compatibility than the diblock formed in situ.
The modulus of elasticity of the starch and PLA blend without MDI was
higher than that of pure PLA, which is typical of a continuous matrix with
a filler. In this instance, the PLA provided a continuous matrix phase,
and the starch served as a filler, resulting in a composite with reduced
strength and reduced elongation which is, therefore, more brittle. In
addition to the modulus of the starch/PLA/MDI blend being significantly
higher than that of pure PLA, the tensile strengths of the blends were
higher or equal to that of pure PLA. In these instances, the starch did
not act as a filler but instead as a compatible polymer through the action
of the MDI.
The crystallinity of the starch and PLA samples blended with 1% MDI was
about 8 times higher than that of the pure PLA (see Table 5) and was about
3.7 times higher after recrystallization (i.e., after the second and third
heating cycles). As shown in FIG. 3 the dynamic storage modulus of the
blend with 1% MDI was higher than that of pure PLA at both room
temperature and above glass transition temperature. The storage modulus of
the pure PLA above its glass transition temperature was about 200 MPa,
which was approximately the consistency of a soft rubber upon removing the
PLA from boiling water. The blend with 1% MDI had a storage modulus above
its glass transition temperature of about 700 MPa, and the blend was stiff
after removing it from boiling water.
TABLE 5
Crystallinity (X.sub.c %) of pure PLA (after molding) and starch and PLA
blends (45:55) with 0.5% MDI (direct blending) as estimated
by DSC measurements.
Samples 1st heating.sup.a 2nd heating.sup.b 3rd heating.sup.b
Pure PLA 7.3 7.1 7.4
Blend 59.25 26.0 23.7
.sup.a Crystals were formed during processing.
.sup.b Crystals were melted and then reformed during cooling in the DSC
pan.
4. Thermal Stability Test
A PLA, starch, and MDI blend was prepared by direct blending as described
in Experiment III, with the weight ratio of starch:PLA being 45:55 and the
blend containing 1% by weight MDI. The prepared sample was then subjected
to a total of four heating cycles wherein during each cycle, the sample
was heated to its melting point, then allowed to cool to room temperature.
Table 6 sets forth the melting points of the blend during each heating
cycle as compared to pure PLA. The melting point of the blend remained
nearly unchanged, through the heating cycles indicating that the inventive
blend had high thermal stability.
TABLE 6
Melting point of pure PLA (after molding) and starch and PLA blends
(45:55) with 1.0% MDI (direct blending) during four heating cycles.
Samples 1st heating 2nd heating 3rd heating 4th heating
Pure PLA 172.degree. C. 171.degree. C. 168.degree. C. 166.degree. C.
Blend 176.degree. C. 175.degree. C. 175.degree. C. 175.degree. C.
5. Reaction Scheme
The structures of PLA, MDI, and an exemplary starch repeat unit are
depicted in Formulas I-III, respectively.
##STR1##
It is believed that the PLA molecule reacts with the MDI molecule as shown
in either Scheme A or Scheme B. In either case, the starch molecule then
reacts with the MDI molecule as also shown in Scheme A and Scheme B.
##STR2##
##STR3##
Example 2
A starch/PLA blend (45/55) was prepared using 0.5% by weight MDA following
the procedure of Experiment III. All of the processing conditions and
procedures were the same as described in Example 1, except that the starch
utilized had a moisture content of about 10% by weight. After the sample
was prepared, the tensile strength and percent elongation were determined
as described in Example 1. The tensile strength of the blend was about 55
MPa, and the elongation was about 4.3%.
Example 3
A PLA starch blend was prepared utilizing hexamethylene diisocyanate (HDI)
as the compatibilizer and following the direct blending method described
in Experiment III of Example 1. The weight ratio of starch:PLA was 45:55
and the percent by weight of HDI was 1%, based upon the total weight of
the starch/PLA/HDI mixture taken as 100% by weight. After the sample was
prepared, the tensile strength and percent elongation were determined as
described in Example 1. The tensile strength was 52 MPa, and the
elongation was 3.51%.
Top